Aluminum alloys have a wide range of applications in light weight structures in aerospace, automotive, marine, wire and cable, electronics, nuclear, and consumer products industries. Among them, aluminum 5000 series alloys are commonly used due to a combination of good mechanical properties and excellent corrosion resistance. 5000 series alloys typically are produced in the form of rolled (sheets, plates) or extrusion products and are utilized in a variety of applications such as automotive body panels, boat and ship body structures, storage tanks, pressure vessels, and vessels for land and marine structures.
An example of an Al—Mg alloy is Aluminum Association 5083 (“AA5083”), which has had a wide range of applications in automotive and marine industries for decades. It possesses a good combination of properties such as high strength, good formability, good weldability, light weight, and low cost. However, a common drawback for this alloy is the susceptibility to inter-granular corrosion (IGC), exfoliation corrosion, and stress corrosion cracking (SCC) and subsequent failure while in service. This phenomenon is called sensitization. Long term exposure of the alloy to moderate temperatures in the range of 80-200° C. can significantly deteriorate the performance of the alloy. An alternative to AA5083 for applications where corrosion resistance is critical is the Al-5454 alloy with 2.4-3 wt. % magnesium. The Mg content in this alloy is reduced to below sensitization critical content (that is at about 4 wt. %). Consequently, mechanical strength of the alloy is reduced; hence, the alloy is not capable of operating in applications where there is a demand for higher strength.
Efforts have been made to improve the corrosion resistance of AA5083 while maintaining mechanical strength. The effect of additions of minor alloying elements such as Mn, Cu and Zn has been investigated. Mn is believed to promote the inter-grain precipitation by providing heterogeneous nucleation sites. Zn is reported to improve the corrosion resistance of Al—Mg alloys by: i) promoting the precipitation of a Mg-phase inside the grains rather than along grain boundaries; and ii) formation of a new ternary phase (so called τ with composition Mg32(Al,Zn)49 along grain boundaries that is discontinuous and has a closer electropotential to the matrix. As a result, higher magnesium content can be tolerated in the alloy. Furthermore, the addition of Cu and Zn together can improve corrosion resistance. Although the mechanism is not clear, it is postulated to be similar to the effect of adding Zn alone. Cu forms Al2CuMg precipitates inside the grain which reduces the formation of a β phase along the grain boundaries. The following references describe some of the efforts in this regard:    King A. Unocic, Paul Kobe, Michael J. Mills, Glenn S. Daehn, “GRAIN BOUNDARY PRECIPITATE MODIFICATION FOR IMPROVED INTERGRANULAR CORROSION RESISTANCE”, Materials Science Forum, 519-521 (2006) 327-332.    M. C. Carroll, P. I. Gouma, M. J. Mills, G. S. Daehn, B. R. Dunbar, “EFFECT OF ZN ADDITION ON THE GRAIN BOUNDARY PRECIPITATION AND CORROSION IN AL-5083”, Scripta Materialia, 42 (2000) 335-340.    M. C. Carroll, R. G. Buchheit, G. S. Daehn, M. J. Mills, “OPTIMUM TRACE COPPER LEVELS FOR SCC RESISTANCE IN A ZN-MODIFIED AL-5083 ALLOY”, Materials Science Forum, 396-402 (2002) 1443-1448.    M. C. Carroll, P. I. Gouma, G. S. Daehn, M. J. Mills, “EFFECTS OF MINOR CU ADDITIONS ON A ZN-MODIFIED AL-5083 ALLOY”, Materials Science and Engineering, A319-321 (2001) 425-428.    Mark C. Carroll, Michael J. Mills, Glenn S. Daehn, Bruce Morere, Paul Kobe, H. S. Goodrich, “5000 SERIES ALLOYS WITH IMPROVED CORROSION PROPERTIES AND METHODS FOR THEIR MANUFACTURE AND USE”, Patent Application Publication No. US 2004/0091386 A1 2004.    Job Anthonius Van Der Hoeven, Linzhong Zhuang, Bruno Schepers, Peter De Smet, Jean Pierre, Jules Baekelandt, “ALUMINUM-MAGNESIUM ALLOY PRODUCT”, Patent Application Publication No. US 2004/0256036 A1 2004.    Job Anthonius Van Der Hoeven, Linzhong Zhuang, Bruno Schepers, Peter De Smet, Jean Pierre, Jules Baekelandt, “WROUGHT ALUMINUM-MAGNESIUM ALLOY PRODUCT”, Patent Application Publication No. US 2004/0261922 A1 2004.
Aluminum 5000 series alloys are typically hardened through two main mechanisms: a) solid-solution strengthening by magnesium, b) strain-hardening by working (H tempers). Consequently, these alloys soften upon exposure to elevated temperatures, due to loss of strain hardening and due to grain growth which hinders their high temperature applications.
Recent efforts have been made to produce Al—Mg alloys that are capable of operating at high temperatures while maintaining other properties such as high strength, high creep resistance, good weldability, high corrosion resistance, and excellent formability. These alloys typically contain a high concentration of scandium. The high price of scandium and limited resources pose limitations to scale up and mass production which is costly for high volume applications. Some of these efforts are summarized below:    N. Kumar, R. S. Mishra, C. S. Huskamp, K. K. Sankaran, “Microstructure and mechanical behavior of friction stir processed ultrafine grained Al—Mg—Sc alloy”, Materials Science and Engineering A 528 (2011) 5883-5887.    N. Kumar, R. S. Mishra, C. S. Huskamp, K. K. Sankaran, “Critical grain size for change in deformation behavior in ultrafine grained Al—Mg—Sc alloy”, Scripta Materialia 64 (2011) 576-579.    N. Kumar, R. S. Mishra, “Thermal stability of friction stir processed ultrafine grained Al—Mg—Sc” Materials Characterization, 74 (2012) 1-10.